Pass a WebContents* to infobar delegate Create() functions/constructors instead
of getting the WebContents from the InfoBarService or using web_contents().  The
former is mainly a simplification, the latter will be illegal once the infobar
refactor lands (and caused the crash in bug 325216).

This will also allow me to subsequently revert r234759 because I can just pass
in the appropriate WebContents*.  (I actually could already do so today, but to
my shame didn't think of doing so.)

BUG=325216
TEST=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/105503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239003 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239003